Until recently, very little was known of the genetics of aggression (intermale fighting) in mice other than its heritability and its dependence on male hormone. Some studies have developed models postulating genetic differences between particular strains and their crosses under controlled conditions of rearing and testing, but none have identified and mapped particular genes to known linkage groups, and few have led to theories of gene action in the development of aggression. However, in a series of papers, we have demonstrated that the DBA/1Bg y-chromsome in the presence of 1/2 its autosomal complement makes an incremental contribution to aggression and that the DBA/1Bg Y-chromsome also is involved in a supranormal pubertal testosterone surge. This pubertal surge in testosterone may be a mediator of the Y-chromosome effect on aggression. Others have demonstrated correlations of Y-chromsome variations with sex ratio or testis weight and investigated the possible relation of the latter to aggression. However, we have obtained no difference between DBA/1Bg and C57BL/10Bg in sex ratio or testis weight, and, although the DBA/1Bg Y-chromosome has an incremental effect on aggression, it does not appear to have an effect on sex ratio or testis weight. This finding may indicate either that there is more than one functional region in the Y-chromsome, or that a single functional region, possibly a gene, has different effects depending upon genetic background. We have also been looking for distinguishing morphological characteristics of the DBA/1 Y-chromosome, which varies in length among different lines. We have shown that the relative length of the Y-chromosome of DBA/1Bg mice is shorter than C57BL/10Bg and that the length of some lines of DBA/1Bg Y-chromosomes increase on the C57BL/10Bg background. This includes the line in which the pubertal testosterone surge occurs and appears correlated with aggressive behavior. These findings, therefore, suggest a correlation between Y-chromosome length and aggression in these stocks, which may or may not be causal. Mice with a shorter Y-chromosome are more aggressive than mice with a longer Y-chromosome within our samples of DBA/1Bg. Congenic stocks are being developed placing the Y-chromosome of each of six genetically defined populations on the genetic background of the other five. These will b (Text Truncated - Exceeds Capacity)